3 on the Tree question

[Grandpas71]

I just bought my first classic truck for restoration. This was my wife's grandfathers truck before he passed away in 1995. It's a 1971 LWB C10 6 Cylinder with 3 on the tree.
Last weekend I removed the gas tank due to no fuel issues. I was thinking the inside of the tank might have turned to varnish since it had sat for so long. So, with the tank out of the truck I thought I would take out the sending unit and float assembly. Found that the "sock" was ripped and the tubing was blocked with muddy looking corrosion. Cleaned it out and installed an inline filter after the tank, runs like a charm!!!

Now I'm wondering what to do with the tranny. Don't get me wrong, my only two cars are straight drives. I love to shift my own. However, my wife only drives autos. I would like to make it more user friendly for her to drive occasionally. And she will not learn to drive a straight drive. But anyways.... What are the dimensions of the current tranny? Model number? Does a turbo 400 slide right in it's place without a drive shaft change? How about a Turbo 350? I'm just curious what's involved, and general information on this three speed tranny.

Thanks,

Grandpas71